An ion implanter includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam typically is mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and implanted into a target. The ion beam may be distributed over the target area by electrostatic or magnetic beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension.
Laser annealing or laser melt annealing may be used to infuse dopants from the ion beam into a workpiece or activate a dopant from the ion beam to form junctions in a workpiece. This workpiece may be, for example, a semiconductor wafer or a solar cell. There are many ways a dopant may be incorporated into the workpiece.
First, solid source drive-in may be used. In this case, the dopant is a solid source at the surface of the workpiece and is driven into the workpiece. Laser energy is absorbed in the surface solid source and is thermally driven into the workpiece below. In some instances, the laser energy also is absorbed in the workpiece and aids diffusion of the dopant into the workpiece and incorporation the dopant.
Second, solid source melt annealing may be used. This is similar to the previous technique in that the dopant is a solid source at the surface of the workpiece. However, in this scenario, the laser energy is sufficient so that the dopant is thermally melted into the workpiece below. Laser energy is absorbed in the solid source and also the workpiece. This embodiment may involve intermixing the melted areas to incorporate the dopant.
Third, implanted source activated annealing may be used. In this scenario, the dopant is implanted into the workpiece, such as using an ion beam or plasma processing apparatus, and then the laser energy is absorbed in the workpiece to thermally activate the dopant or incorporate the dopant into the workpiece.
Fourth, implanted source melt annealing may be used. This is similar to the previous technique in that the dopant is implanted into the workpiece using an ion beam or a plasma processing apparatus. Laser energy of a sufficient energy is absorbed into the workpiece to thermally melt the workpiece so that the dopant and workpiece are mixed together and recrystallize together.
Laser energy absorption is one factor that affects annealing or melting to form junctions. Absorption properties of the silicon workpiece dictate the type or wavelength of the laser used to melt or anneal. FIG. 1 is a chart illustrating absorption depth of silicon versus wavelength of a laser. To form junctions with a depth of approximately 1 μm, short lasers such as excimer lasers are typically used. For thin workpieces, such as those <200 μm in thickness, many lasers with higher wavelengths (e.g., >400 nm lasers such as 850 nm diode, 1.06 μm YAG, 10.6 μm, CW lasers, fiber lasers) may not be absorbed in the workpiece because the workpieces may be transparent to the wavelength. Thus, such lasers may not be appropriate to melt junctions in these workpieces.
Forming junctions through melting has several advantages for workpieces including solar cells. Melting enables lowest resistivity for a given dopant density and the highest possible activation. In some cases, 100% activation may be achieved. Shallow and abrupt junctions may be formed and there may be less dopant loss due to segregation. Thermal budget is reduced for the rest of the workpiece because this remainder of the workpiece is not at silicon melt temperatures. Thus, workpiece quality degradation due to an increase in thermal budget is avoided. Furthermore, if the workpiece is a solar cell, different solar cell architectures may be enabled.
Laser melt anneals typically use short wavelength lasers such as excimer or ruby lasers. Junction depths are controlled by increasing incident laser power density, which may introduce damage into the workpiece. These short wavelength lasers, however, may be too expensive for use in manufacturing. Using lasers of longer or higher wavelengths would be more economical. FIG. 2 is a chart illustrating cost of ownership (CoO) and cost of consumables (CoC) versus wavelength. Longer wavelengths are comparatively less expensive and cheaper to maintain. Accordingly, there is a need in the art for a method that uses longer wavelength lasers to form shallow melt junctions.